Although researchers at McGill University in Montreal, Canada, said sufferers should not stop taking the drug they recommended that it be supplemented by a further anti-coagulant during the first month.

“There is no question that warfarin is highly effective in preventing strokes in patients with atrial fibrillation,” said Prof Laurent Azoulay of the Department of Oncology at McGill University.

“Thus, our finding that the initiation of warfarin may be associated with an increased risk of stroke should not deter physicians and patients from using this drug, since this likely affects a small number of patients.

“However, the results of our study suggest that physicians should be vigilant when initiating warfarin, particularly in the first week of use," he said.

Atrial fibrillation occurs when electrical impulses in the upper chambers of the heart, called the atria, become chaotic and cause an irregular heartbeat.

The irregular heartbeat can result in heart palpitations along with a variety of symptoms such as fatigue.

When the heart is not pumping blood effectively, blood can stagnate and clot. If the clots break apart and travel to the brain, they can cause a stroke.

The researchers analysed data from 70,766 patients aged 18 or over, who were diagnosed with atrial fibrillation between 1 January 1993 and 31 December 2008

Atrial fibrillation increases the risk of patients suffering a stroke, and warfarin is frequently prescribed for these patients to reduce the likelihood of this happening.

During that time, a total of 5519 patients experienced a stroke (two percent per year).

During the first 30 days after starting warfarin, there was a 71% (nearly double) increased risk of ischaemic stroke when compared with patients taking no anti-coagulant drugs.

Once the first 30 days had elapsed, the risk of a stroke was halved in patients taking warfarin compared to non-users, researchers found.

Previous studies have established that there is a possibility that warfarin leads temporarily to a ‘hypercoagulable state’ – one in which the blood becomes more sticky and more likely to develop dangerous blood clots in the veins or arteries.

Professor Samy Suissa suggested that a "a bridging strategy using heparin – an injectable anticoagulant – at the initiation of warfarin treatment could be considered as a way to reduce the increased risk observed in the first 30 days of use."

Dr Peter Coleman, Deputy Director of Research at Stroke Association, said: “Anticoagulants, such as warfarin, have long been used to reduce the risk of stroke in people with an irregular heartbeat by preventing clots from forming in the heart. These clots can travel to the brain causing strokes.

“Whilst warfarin is undoubtedly extremely effective in preventing strokes in people who have atrial fibrillation, it is not an appropriate treatment for everyone. Patients who are prescribed warfarin must be monitored by regular blood tests to ensure that the drug is working safely and effectively. It is vital that patients taking warfarin attend all tests advised by their doctor to ensure that their dosage is correct.

“There are new alternative treatments for people who are unable to take warfarin to prevent their risk of stroke. Anyone at all worried about their medication should speak to their GP to discuss the treatment option that works best.”